362 CHEMICAL ENGINEERING
number of tests on experimental evaporators with copper tubes, and the average results are represented by the formula
Ko = 225 -f 17,5001)
where D is the density of the heating steam in pounds per cubic foot. For steam at atmospheric pressure (212°F.), K0 would be 925; for a vacuum of 20 in. which usually prevails in the steam chest of a fourth effect, K0 would be 450. Both these figures are much higher than the results obtained in actual practice.
The coefficient of heat transmission also changes with the density and viscosity of the liquor, and the reduction is usually in direct proportion to the specific gravity. Impurities and suspended matter will reduce the factor, and high velocity and good distribution of the steam will greatly increase the heat transmission which is evident in evaporators with properly proportioned steam coils which sometimes show exceedingly high capacities per square foot of heating surface. The heating steam always contains air and non-condensable gases which greatly reduce the heat transmission, as the air seems to form an insulating surface on the tubes. It is therefore of the utmost importance to provide efficient means of removing air and gases from the heating element. Steam gages will show the total pressure of steam and air in the steam chest, but the actual temperature is only due to the pressure of the steam and should therefore be determined by a thermometer and not from the gage pressure. According to Orrok, K0 varies as (ps/pt)* where ps denotes the pressure of the steam alone, and pt the total pressure (= ps + pressure of air).
The condensed steam forms a layer of water on the heating surface, and it is necessary especially in coils and horizontal tube evaporators that this condensate is removed as quickly as possible to avoid reduction in capacity.
Rapid circulation of the liquid and a low liquor level will greatly increase the capacity. A thin film will naturally cause a rapid evaporation, and it has been found that in all vertical-tube evaporators the highest capacity is obtained by keeping the liquor level at about one-third of the length of the tube above the lower flueplate. The upper part of the tube is then covered only by a thin film which evaporates very quickly. This method can, however, not be used in cases where salts are separated from the liquid, as these salts would naturally form a coating on the tubes. For further details regarding influence of liquor level see E. W. Kerr, Bull 138 and 149, Louisiana State University.
It has been shown that the actual amount of heat transmitted will depend on a great many factors and will vary considerably with the type of apparatus and the kind of liquor. Outside of water distillation, the extreme limits in actual practice are probably 8 Ib. per hour per square foot for electrolytic caustic liquor from 25 to 48° in a vertical-tube evaporator and 62 Ib. evaporation per hour per square foot for malt extract from 5 to 30°B<§. in a rapid circulation film type evaporator with steam at 5 Ib. and a vacuum of 28 in. Practical results of various liquids are given in later paragraphs, p. 375 et seq.
Evaporation Under Vacuum.—Boiling under reduced pressure has the following decided advantages: Increased temperature difference; lower initial steam pressure; low boiling temperature of the liquid; reduced steam consumption. An increased temperature difference means a higher capacity per unit of heating surface and therefore a smaller heating surface for the same output. A vacuum evaporator is, however, more expensive than the atmospheric type, as it requires a closed vessel with large vapor space and a condensing system.
A great economy is often accomplished by utilizing waste steam of engines and pumps instead of high-pressure direct steam, and the increased cost of equipment is frequently paid for in a few months by the saving in coal.